Approximately one half of all Americans living with Hepatitis C virus (HCV) are drug users, yet they are the least likely to receive HCV treatment. Drug users are presumed non-adherent and therefore denied potentially life-saving therapy. This assumption can only be confirmed or dispelled through prospective pharmacologic and adherence studies in this population. Such studies would be greatly enhanced by an objective, quantitative measure of adherence which does not currently exist in the HCV field. Through the work proposed in this application, sixty HIV/HCV co-infected drug users will be treated with direct acting antiviral agents (DAA) and randomized to receive directly observed DAA therapy (DOT) vs. no directly observed therapy (no-DOT). Patients randomized to no-DOT will have wirelessly observed therapy (WOT) which involves use of a portable medication dispenser that sends a signal to a server with the date and time when the dispenser is opened. In Aim 1, DAA concentrations will be compared in those randomized to DOT vs. no-DOT. DAA pharmacokinetics will also be defined accounting for clinical factors like degree of hepatic impairment and use of concomitant recreational and antiretroviral drugs. The goal is to quantify adherence in this population and the effect of variable adherence on drug concentrations. In Aim 2, DAA concentrations (plasma, cellular, hair) will be linked with adherence patterns identified using WOT and DOT. The goal is to identify a drug concentration biomarker that predicts adherence in this population. In Aim 3, the relationship between DAA adherence (as measured by WOT and DOT and drug concentrations) and rate of cure will be established. The goal is to define the degree of adherence needed for HCV cure. This project is important to human health as it will treat a neglected patient population, use technology to make adherence monitoring convenient, evaluate novel, pharmacokinetic-based adherence measures, determine the contribution of adherence to the likelihood of HCV cure, and generate the first data on DAA forgiveness. The work proposed will generate much needed pharmacology and objective adherence data to encourage the treatment of HCV in drug users.